Space discharge apparatus and circuits therefor



A. M. SKELLETT 29205,@21

SPACE DISCHARGE AIPARATUS AND CIRCUITS THEREFOR Filed July 31, 1936 3Sheets-Sheet 1 [Zizas f/6\ //0 F/G.

ooio'ooooooooo /NVENTOR A M. SKELLETT )IMT/m Jun@ E89 IW, A, M SKELLETT2,205,07I

SPACE DISCHARGE APPARATUS AND CIRCUITS THEREFOR Filed July 5l, 1936 3Sheets-Sheet 2 F/G I 38 l 50 53 4o' Ch'- {EEES-:55M V L52 x -lvlnln :l

im@ 1li A. M. SKELLETT 2,205,073

SPACE DISCHARGE AIPRATUS AND CIRCUITS THEREFOB Filed July 5l, 1936 3Sheets-Sheet 3 F G. 8 I3 u f7.5 /86 f77 f77 /Nl/ENTOR A. M SKELLETT ByJM Patented June 18, 1940 UNITED S ATES TENT FFECE SPACE DISCHARGEAPPARATUS AND CIRCUITS THEREFOR Application July 31,

13 Claims.

The present invention relates to space discharge apparatus and circuitstherefor, in which use is made of secondary electron emission producedby successive impacts of electrons on emissive surfaces. Devices of thissort are commonly termed electron multipliers, and will be so referredto in this application.

Representative objects of the invention are to improve upon thestructure and mode of operation of such apparatus and upon the circuitsused with such apparatus.

More specifically, it is an object of the invention to provide a circuitin which the operation of `an ,electron multiplier is stabilized.

Another object is a duplex or push-pull type of electron multiplier, thetwo sides of which preferably operate in alternation.

The various objects and features of the invention will be made moreapparent from the following detailed description of representativeembodiments of the invention as illustrated in the accompanying drawingsin which Fig. l is a schematic diagram partly in section of a tubestructure and circuit iereior in accordance with the invention;

Fig. 1A shows a detail of the tube structure of Fig. l; v

Fig. 2 is an alternative type of tube structure and circuit therefor inaccordance with the invention;

Figs. 3 and 4 show alternative types of electron sources which may besubstituted for the correspending portions of either Fig. 1 or Fig. 2;

Fig, 5 shows an electron multiplier tube structure and circuit thereforof a different type in accordance with one form of the invention;

Figs. 6 and 7 are detailed showings of a type of electron sourceillustrated in the structure of Fig. 5 but also capable of general use;

Fig. 8 is a longitudinal View oi a modified type of tube structure inaccordance with the invention embodying a stabilizing feature oi theinvention;

Fig. 9 is a cross-sectional view taken along the line 9-9 with somedetails omitted;

Fig. l0 shows a curve to be referred to in connection with thedescription of the operation of Fig. 8; and

Fig. 11 shows a view of a stabilizing circuit that may be used in lieuof that of Fig. 8.

It is known that electron multiplier amplifiers have proved effective inproducing very large `amounts of amplification and that their operationcompares favorably with that of conventional vacuum tube amplifiers,particularly with 1936, Serial No. 93,536

respect to thenoise level. So far as known, one of the most effectiveways of producing the electrons initially for subsequent multiplicationby such apparatus has been by means of a photoelectric cathode Where thesignal or other variations to be amplified is in the form of a modulatedlight wave. This method of producing the initial electrons appears tohave serious limitations when the signal or other variations to beamplified are in the form of small currents pro- 10 duced outside of theelectron multiplier tubes.

It has, of course, been suggested that` the photoelectric emitter at theinput might be replaced by a thermionic cathode and a grid. A seriouslimitation to this simple change is that the modulation of the directcurrent to the first multiplier stage is likely to be ,extremely smalland since this modulation is constant throughout the multiplier thedirect current component at the output will often be large in comparisonwith the signal. For useful operation, the electron current to the iirstmultiplication stage should be modulated by the signal to a degreeapproaching as nearly as possible 100% modulation. This means that wherea cathode and grid or any other type of electron emitter and inputcontrol are used, they should operate in analogous manner to theso-called class B or class C operation of the conventional amplier.These considerations have led the applicant to devise electronmultiplier apparatus working on the duplex or pushpull principle,typical forms of which will now be described with reference to thedrawings. i

Referring first to Fig. l, an elongated evacuated envelope Id is shownas provided at its left-hand end with suitable electron emitter andinput elements to be described, with a central row of emitters il, I2,I3, etc., an upper row of deflecting plates I5 and a lower row ofdeflecting plates I7. The emitter plates, II, l2, I3, etc. are suit- .40ably coated on both their upper and lower surfaces to promote liberationof secondary electrons under electron impact, such as by depositing`caesium on an oxidized plate of silver or in any other suitable manner.In the right-hand end .45 portion of the envelope I6 are anodes I8 andI9 separated from the rest of the structure by a grid or shield grid 2lipreferably extending all the Way across the tube. Anodes I8 and I9 areconnected to the primary of output transformer 2|. Each .50 emitterplate II, l2, I3, etc., is connected to the corresponding deflectingplate of the upper row IG and of the lower row II and to a point on thepotentiometer 22 which is supplied with suitable terminal voltage asindicated. It will 55 be understood that each emitter plate and itscorresponding deflector plate are at a more positive potential than theset of emitter and deflector plates immediately preceding them.

In the structure illustrated in Fig. 1 the electron emitter and inputstructure comprises heating filaments 24, 24 supplied with heatingcurrent from battery 25, and cathodes 25, 25 which are heated bythefilaments 24, 24 to render them suitably emissive. Grids 21, 21 arepositioned between the cathodes 25, 25 and the multiplier" chamber andare connected yto the terminals of input winding 23. A shield orscr'een28. extends across the tube and divides the structure into twoportions except for the windows illustrated at 30, 30. The detailedFigure 1A shows the general shape and position which these aperturesl3l), 30 in the plate 28 may have. Fig. 1A also shows a pair ofpermanent magnets 3l and 32 situated with respect to tube lli so astoproduce fa magnetic eld in the upper portion of the tube, opposite insign to that produced in the lower portion. of the tube. No attempt hasbeen made to show these magnets in connection with Fig. 1 since it wouldneedlessly complicate the drawings. It is'to-be understood, however,that preferably a number of magnets such as 3l will be positioned alongtube I0 and a similar series of magnets 32 Will also be positioned alongthe tube I9 so as to pr'oduce the oppcsitely directed magnetic fields inthe upper and lower halves of the tube throughout the multiplier portionof the device.

In the 'operation of the structure and circuit 'according to Fig. 1, thegrids 21, 21 are polarized by the connection of conductor 29 to thepotentio- :meter .22 by a sufficient negative voltage r'elative kto thecathodes 25, 25 so that in the absence of applied waves through theinput'coil 23, no electrons are permitted to pass through the windows30, 39 to the multiplier portion of the apparatus. YIt will be notedthat a relatively high positive v:voltage is applied to the plate 28 soas to attract :electrons towards the apertures 39, 39 when eitherVgrid'21, 21 has its voltage sufciently shifted in `the positivedirection. When signals are applied from the injut circuit through theinput coil 23,

vone grid 21 -is instantaneously driven positive `while the other one isdriven more negative. When thev variation applied to one of the grids inthe positive direction sufciently overcomes vthenorrnal bias, electronsare permitted to pass through the `corresponding aperture and enter theelectron chamber. Under the iniiuence of themagnetic field theseelectrons are deflected vtowards the plate Il which they strike atsufli- I-cient velocity to release a larger number of secaondaryelectrons. These secondary electrons are Aconstrained by the magneticfield and by the higher positive potential of the next set of emitteranddeflectorplates ,(towards the right in '1Fig.1) to follow a curved lpathand strike the 'second emitter plate l2 from which they release YThisprocess continues throughout the length of the multiplier portion of thetube, the secondary electrons emitted from the final plate I5 passingthrough the screen grid 29 to either anode i8 or *19- as the case maybe. Thus they upper and -lowerportions of the multiplier structure come'into play alternately under control of the input wave and produceamplified current flow to either anode I8 or I9. The amplified outputcurfrents are transmitted throughthe output coil 2| 'into the outgoingcircuit.

'Ih-e invention is notrlimited to the case in which the two sides orportions of the electron multiplier structure are included within thesame envelope nor to the case in which one set rof emitter plates Il,l2, i3, etc., is used in common by both portions of the electronmultiplier structure since other' constructions will suggest themselvesto those skilled in the art.

Referring to Fig. 2 the electron emitter and input structure may be thesame as that of Fig. 1, corresponding elements being indicated by thesame reference characters. In this case the envelope is represented at33 and the multiplier portion of the apparatus is comprised between vthevertical plates 38 and 39 and the horizontal plates 4l and 42 positionedgenerally as shown. A central shield member d0 extends lengthwise rofthe chamber and preferably entirely across it.

The end plate members 38 and 39 have applied to them a high frequency orradio frequency voltage from source 3 connected to transformer 44.Shield i9 is maintained neutral with respect to this high frequencyvoltage by being directly .voltage to plate 38 relative to cathodes 25,25. In

this figure, as in Fig. 1, the grid bias is maintained at sucnhighnegative Value that in the absence of input waves to Abe Vamplified noelectrons are permitted to enter the electron multiplier chamber throughthe apertures 30, 30.

When the potential of either-grid 21 varies sufficiently in a positivedirection to permit electrons to enter the corresponding aperture 38,the electron comes under the inuence of the high frequency field and iscaused to traverse a path to and fro roughly as indicated in dottedlinnes in the drawings, supposing that the high frequency field is atthe moment under consideration such as to make the end plate 39 positivewith respect to plate 33, the electron is accelerated towards the rightin the figure, that is towards the plate 39. It will be understood thatplates 38 and 39 are suitably coated to render them emissive to liberatesecondary electrons under electron impact. At a suitable instant thehigh frequency eld reverses in sign so that plate 39 becomes negativewith respect to plate 38. The electron in question, however, has beenaccelerated so that `it strikes the plate 39 driving out a larger numbervof l secondary electrons which are immediately driven toward the leftin the figure, that is toward the plate 38 under the influence of thehigh frequency electrical field.l At the same time these electrons areattracted toward either plate 4l or 42, as the case may be, because ofthe high positive voltage existing'on these two plates. Theresultantfpath is the zigzag dotted line referred to whereby after asuccession of impacts on the plates 38 and 39 the secondary electrons,greatly increased in number, finally reach the plate As inthe case ofFig. 1 the upper' and lower parts of the multiplier chamber of Fig. 2act in alternation, depending upon the reversal of sign of the inputwaves through transformer 23. The amplied output flows alternatelythrough the upper and lowerfhalves ofthe primary of the outputtransformer 2| and induces amplified outlput voltage in the outgoingcircuit.

-The foregoing description hasassumed that ther'e was not necessarily`any magnetic eld in the tube. Generally improved results are attainedby use of a magnetic field which may be produced for example by applyingdirectcurrent to the winding 34 surrounding the envelope 33. 'I'he neldso produced is in the horizontal direction parallel to the plane of thepaper'. This eld causes the electrons to take a spiral rather thansimply a zigzag path and prevents them from straying across to anodeslll, 42 from their points of emission on the surfaces of plates 38, 39.The back and forth lines of night in Fig. 2 should in this case bepictured as the edge-on view of a spiral.

Referring now to Fig. 3, the structure shown in this gure is alternativeto and may be substi- Y tutedfor the portion of the structure off-Fig. 1

lying to the left of the broken line 3 3. This alternative structurecomprises at the left-hand end of the tube I an electron gun, generallyindicated at 50, of known construction for producing a concentrated beamof electrons passing between the deiiector pla-testi! and 52. Thedetails of the electron gun structure are not shown but may be, forexample, of the type disclosed in U. S. Patent 1,632,080 to J. B.Johnson, June 14, 1927, or any other suitable type. In the absence ofinput waves, that is, in the normal condition, the electron beamimpinges on the central portion 53 of the plate member 28 so that noneof the electrons enter the multiplying chamber through either aperture30, 36. It is desirable that the edges of the electron beam be made assharp as possible and this may be attained by suitable electron focusingin a manner known in the art. Signal waves applied to the inputtransformer 23 vary the potentials on the plates and 52 causing the beamto be swung upward cr downward in the gure so that a variable portion ofthe beam, depending on the signal amplitude, enters one or the otheraperture 30, 30.

In order to prevent secondary emission from the plate 23, particularlythe central portion 53, the surface may be coated with carbon or,alternatively, a shield (not shown) may be placed in front of the plate2B. 'Ihe position of the field magnets (see Fig. 1A) is such that thereis but small stray field in the portion of the apparatus shown in Fig. 3capable of affecting the direction of the electron beam. If founddesirable, however, in any case magnetic shielding can be used for theportion of the device shown in Fig. 3. For example, member 28 could bemade of magnetic material and could be supplemented by iurther' magneticshielding within or without the tube. Since the electron velocity ishigh in this portion of the device any existing stray field would haveless eifect on the beam direction than it would on the electrons insidethe multiplier.

The type of electron source shown in Fig. 3 possesses advantages overthat shown in Fig. 1 for certain uses, for example for relatively smallinput potentials. If the input potentials are too small, the velocitydistribution of the electrons in the structure of Fig. l might be suchthat comparatively few electrons are passed into the multiplicationchamber. With the structure of Fig. 3 the electron velocity may be madehigh regardless of the'value of the input potentials.

The structure shown in Fig. 4 is alternative to and may replace theportion of the structure of Fig. 2 Shown to the left `of the broken linelim-4. In view of the description given of Fig. 3 no detaileddescription of 4 is deemed necessary since the drawings, together withthe reference characters, make it clear how the substitution of thisstructure in the structure of Fig. 2 could be made.

In the structure of Fig. 5 which represents a plan View of theapparatus, envelope 5B is in the form of a squat cylinder including anouter row of emission plates 60 in the form of arcs of a circle and aninner row of deiiector plates 59 also arranged in a circle concentricwith plates 00. The progress of the electron stream is indicated by thedotted arrow line as extending from the electron source 64 (to bedescribed later) against the inner surfaces of the emission plates 60 insuccession around the device and finally to the output plate or anode 6Iin front of which is a screen grid 62. In this figure the magnetic eldis normal to the plane of the paper as in-n dicated by the curvature ofthe electron paths. Each pair of plates 59, 60, extendingcounterclockwise around the device, is maintained at higher potentialthan the preceding pair by suitable connection to points onpotentiometer 22. A centrally positioned shield 63 prevents directpassage of electrons across the device from the relatively low potentialplates 59, 60 near the input of the device to the relatively highpotential plates near the output side of the device.

'I'he initial electron source 64 consists of a small thin lm of metaladapted to be variably heated by the input waves and coated on itssurface with suitable electron emitting material. This may beconstructed as shown in Figs. 6 and '7, of which Fig. 6 represents aplan view and Fig. 7 a section. A mica support 69 has a central aperture'lll through it and is covered over with a thin sheet of celluloseacetate 68 on the top of which is deposited a thin layer of metal 6i,applied by sputtering the metal or evaporating it on the surface of theacetate lm. The metal is then activated to render it thermionicallyemitting. The metal film 64 is cut away at its middle portion as shownin Fig. 6 so that only a very narrow portion lies over the aperture 1U.This type of structure has been used by applicant in making thin lmthermocouples and applicant has found that thermocouples constructed inthis Way are capable of responding to very small temperature variationsat frequencies up to and beyond 20,000 cycles per second.

Referring again to Fig. 5, the electron emitter 64 is supplied with anormal or biasing current from battery 65 through regulatingpotentiometer 66 to produce a mean value of heating current and acorresponding mean value of emitted electrons. Speech waves or othersignal waves applied through input transformer 56 cause instantaneousincreases or decreases in the temperature of the element 64 withcorresponding increases and decreases in the number of electrons emittedfrom instant to instant and these variations n the number' of electronsemitted are amplified by the multiplier apparatus in a manner that willbe obvious in View of the description of the previous figure. For somepurposes the battery 65 and resistance 66 may be omitted.

Amplifying devices employing electron multiplicati-on have thecharacteristics of ct current amplifiers. Also the more succe nil onesto date work under conditions of voltage saturation in so far as theemitter surfaces are concerned. In other words the emitting surfaces donot have a region of heavy space charge immediately above them; theemitted electrons are drawn off by a positive potential gradient at thesurface. Now it is well known to workers in the art that a region ofspace-charge nextto an electron emitting surface, particularly thosecomposite surfaces that are most eiiicient as electron emitters, has astabilizing effect on the emission. Thus the electron multiplier of theusual sort when utilized as an amplifier does not have as good stabilityas the more conventional types of ampliiier and means for stabilizingsuch devices would be very useful. Such stabilization is secured bymeans of the present invention in two general ways as will now bedescribed. The first method is by causing the output current to react onthe magnetic iield 'used for focusing the electron stream, and thesecondmethod is by causing the output current to vary the direct supplyof voltage applied to the emitter and deiiector plates of the device.

The first of these two methods is illustrated in connection with Fig. Sin which an elongated tube 15 includes a series of secondary emitterplates 16 and a series of deflector plates 11. left of the aperturedplate 18 any suitable type of primary electron source may be used, thatshown being the electron beam type generally similar tol that describedin connection with Figs. 3 and 4. Input waves applied through inputtransformer 19 apply varying potentials to the beam deflecting plates 80and 8l causing more or less of the electron beam to enter the aperture82 and impinge on the rst secondary-emitter plate 16. It will beunderstood that the aperture 82 is positioned so as normally to lieslightly to one side of the electron beam. Secondary electrons arereleased from the successive emitter plates 16 as in the previousfigures andl those from the nal plate 1S pass throughy the screen 83 tothe output or anode plate B producing Variable current iiow in theprimary winding of output transformer 95.

The position of one pole face ofthe focusing magnet is indicated bydotted'lines at in Fig. 8 and the structure of the magnet itself will bemore apparent from the sectional view of Fig. 9. The magnet 81 havingpole faces', 36 may be either apermanent magnet or an electromagnet. Inthis case it is assumed to be a permanent magnet with a superposedwinding 90 for varying the strength of the magnetic field. This winding90 is connected between the primary winding of output transformer 85 andthe positive pole 93 of the source of supply voltage 'shown connectedbetween terminals 92 and 93. A relatively large capacity 3Q in shuntwith winding 99 permits current variations of signaling' frequency toreturn directly to the supply source.` Relatively slow variations in theoutput current representing instability in operating characteristic passthrough the winding 90 and vary the Strength of the focusing field ofthe magnet 81 to react on and change the value of the output current invdirection and amount to improve' the `stability of operation of thedevice.

The action is illustrated by Fig. 10 which shows a typical curve plottedbetween strength of magnetic field and magnitude of output current. Byoperating on the steep portion of this curve at either point A ory pointB a small change in strength of magnetic iield produces a relativelylarge change in output current and it is only necessary to observe aproper relation between direction of winding of the coil 99 and thedirection of the effect of change of magnetic field on output current tosecure a compensation for instability by this means.

The second method of correcting for instability TO thev may beaccomplished by the apparatus shown in Fig. 11 which may replace theportion of the apparatus in Fig. 8 shown to the right of the broken lineH--I I. In this case no use is made of the coil 90, the terminals ofwhich may therefore be left open. The positive pole 93 of the voltagesupply source leads to the anode of a three-element tube 91, the cathodeof which is connected in series through a resistance 95 and primarywinding of output transformer 85 to the anode 84 of the tube 15.Resistance 95 is bypassed by condenser 96. Variations in output voltageof signaling frequency produce no eifect on the tube 91 since they arebypassed by condenser 96. The grid of the tube 91 is biased by currentiiow through resistance 95 which may be supplemented with other biasingmeans if desired or necessary to give the tube 91 a normal internalresistance of appropriate value. Relatively slow variations in outputcurrent representing unstable operation develop potential differencesbetween the terminals of resistor 95 which are applied to the grid ofthe tube 91 and` vary its internal impedance and consequently thevoltage supplied to the potentiometer 22 from which the voltage is takenoff for the various emitter plates.

It is found that the supply Voltage characteristic ofthe device is ofthe same general shape as the curve of Fig. l and by working on the lowvoltage side of the characteristic, that is, where is positive, thecircuit of Fig. 1l will provide stabilizing action. If desired, the tube91 may be a screen grid or pentode type of tube or any other suitabletype.

` In operation a small decrease in the amplification of the device 15will cause a corresponding decrease in the output`current throughresistance 95 andrtube 91 causing the grid of the tube to goI morepositive and the internal resistance of tube 91 to be lowered. Morecurrent will then flow through the potentiometer 22 raising the voltagesupplied to the electron emitter plates and increasing the amplificationup to its normal value. An increase in amplification above normalresults in similar manner in a decrease in the supply of voltage andstabilization at the working value.

Either of the two stabilizing methods illustrated in connection withFigs. 8 to 1l may be used in connection with the circuit structures ofany of the other figures in which a magnetic iield is used, or in thecase of Figs. 2 and 4 if a magnetic field is not used to assist infocusing the electron beam, the second of the two stabilizing methodsmay still be used. For example, in connection with Fig. 1 it is onlynecessary to include a suitable winding, such as winding 90 of Fig. 8,.on each of the field magnets 3|, sil (of Fig. 1A) and to connect themwith proper polarity in the output lead 2|. Alternatively, atube similarto tube 91 of Fig. 11 may be connected between the positive end of thepotentiometer 2'2 of Fig. l and the positive terminal of the supplysource, with the resistor 95 of Fig. 11 connected in the anode lead 2 Iof Fig. 1. It is believed that it willv be obvious from this descriptionhow the stabilizing methods may be applied to the other figures. l

The stabilizing circuits and methods are claimed lin my copendingapplication Serial No. 167,525, filed October 6, 1937.

It will be understood that the various structures and circuits disclosedherein are to be taken as illustrative rather than as limiting and thatvarious modifications and departures from the detailed disclosure may bemade within the spirit and terms of the appended claims.

What is claimed isz' l. In, space discharge apparatus a duplex electronmultiplier having an exciting stage comprising a source of electrons andmeans to control by. waves to be ampliiied the admission oi electrons tothe electron multiplier, said means permitting electrons to enter eitherside of the electron multiplier only when input waves are incident uponthe apparatus and then to enter only one side of the multiplier at atime depend* ing upon the polarity .of the input wave at the particularinstant, means preventing primary electrons entering the electronmultiplier from passing directly to the multiplier output, said controlmeans comprising a pair of control elements, an input circuit connectedbetween said elements, and an electrical connection between a point ofelectrical symmetry in said input circuit and a point of electricalsymmetry in said duplex electron multiplier for. maintaining saidapparatus in electrical symmetry.

2. In space discharge apparatus, an electron multiplier structurecomprising two symmetrical portions, a common output circuitdiierentialli' connected to said two portions, means te initiateelectron discharge in the inputs of said portions in alternate timeportions respectively of an input control wave, means to prevent primaryelectrons from passing directly from the said inputs to said outputcircuit, an input circuit differentially related to said two portionsand a connection between said input circuit and said elec* tronmultiplier structure for maintaining said input circuit in electricalsymmetry with respect to said multiplier structure.

3. In space discharge apparatus, a duplex electron multiplier structurecomprising symmetrical halves with means in each half for producingmultiplication of electrons by successive impacts with secondaryemission plates, a plurality of said plates being common to both halvesof said device, means to apply input variations diiierently to said twohalves, and an output'difierentially connected to said two halves.

4. Apparatus according to claim 3, including means preventing electronflow in either half of said structure in the absence of applied inputvariations.

5. Apparatus according to claim 3 in which the electron multiplierstructure comprises a succession of plates positioned along the axis ofthe device, said plates being rendered suitably emissive on theiropposite surfaces, and means in eachv half of the structure causingimpacts of electrons on opposite sides of said plates.

(i. In a duplex or push-pull amplier, an evacuated enclosure comprisingan electron multiplier structure divided into two portions, an outputelectrode in each portion, an output circuit diiierentially connected tosaid output electrodes, a shield containing apertures admitting to eachportion of said multiplier, an electron omitting cathode in front ofsaid shield, a pair of electron control members between said cathode andsaid shield, an input circuit diierentially connected to said controlmembers, a circuit conductively connecting said shield, cathode and themid-point of said input circuit, means normally biasing said controlmembers in a direction to prevent passage of electrons through either ofsaid apertures and to such an extent that in the absence of input waves,no electrons are permitted to pass into the electron multiplier, meanscoating the surface of said shield rendering it a poor emitter ofsecondary electrons, and means preventing electrons that pass throughsaid shield from traveling directly to a said output electrode.

'7. Apparatus according to claim l in which the preventing meanscomprises means for providing a magnetic eld for preventing primaryelectrons entering the electron multiplier from passing directly to themultiplier output.

Apparatus according to claim 2 in which the means to prevent comprisesmeans for providing a magnetic eld for preventing primary electronsentering the electron multiplier from passing directly to the multiplieroutput.

9 l'n space discharge apparatus a duplex electron multiplier structurecomprising a chamber having opposite walls and separated by a centralshield into two portions. means to apply input variations diierentiallyto said portions, means rendering said walls suitably emissive, means toapply an alternating field to said walls to cause successive impactsagainst said walls, an output plate in each portion, and meansmaintaining plate positive with respect to said central shield.

l0. A symmetrical duplex electron multiplier including two parallelbranches each branch containing a series of multiplier stages, a commonconnection for the initial stages, an output electrode in each branch,an output circuit connested across said output electrodes, an excitingstage comprising a source of electrons and a pair of control elementsfor controlling admission of electrons to said branches alternativelyunder control of input waves, an input circuit connected across saidpair of control elements, an, electrical connection between themid-point of said input circuit, said common connection for the initialelectron multiplier stages and the mid-point of said output circuit forpreserving the electrical symmetry of the multiplier, and meansassociated with the multiplier stages in said branches for preventingdirect passage of primary electrons entering said branches from passingdirectly to said output electrodes.

ll. A space discharge device comprising a signal input portion and anamplifying portion consisting of two electron multipliers each includinga series of emitting surfaces and means for causing electronmultiplication by successive impacts of electrons against said surfaces,said sig nal input portion comprising a source of electrons and meansfor causing electrons therefrom to impinge on the rst emitting surfaceof one or the other of said multipliers alternatively under control ofimpressed potentials, depending upon the polarity of the signal at aparticular instant. said last means including a pair of input controlelements, a signal input circuit having its opposite terminals connectedto said input control elements, and an electrical connection from themiddle point of said signal input circuit to the first emitting surfaceof each of said multipliers for xing the average potential of said pairof input control elements symmetrically with respect to said emittingsurfaces and to said signal input circuit.

l2. In space discharge apparatus, a duplex electron multiplier having anexciting stage comprising a source of electrons and means to control byinput waves to be amplied the admission of electrons `to the electronmultiplier, said means permitting electrons to enter either side of theelectron multiplier only When input Waves are incident upon theapparatus and then to enter only oneY side of the multiplier at a timedependingupon thepolarity of the input Wave `at the particular instant,said means comprising a pair of control elements, an input circuithaving its opposite terminals connected to said control elements andhaving a center tap point, connections from said center tap point tosaid source of electrons and'to a point of electrical symmetry in saidduplex electron multiplier, said connections maintaining the `apparatusin electrical symmetry.

- 13. In space discharge apparatus a duplex elec tron multiplier havingan exciting stage comprisinga .source of electrons and means tocontrolby -Waves to be amplied the admission of electrons to theelectron multiplier, said means permitting electrons to'enter eitherside of the electron multiplier only when input Waves are in-rcidentupon the apparatus and then to enter only one side of themultiplier at a -time depending upon the polarity of the input Wave atthe particular instant, said multiplier comprising in each of its twosides a succession of electron emitting surfaces, means to applysuccessively higherfpositive potentials to said vsurfaces and meansdirecting electrons toward the successive emitter surfaces, said controlmeans comprising a pair of control elements, an input circuit connectedbetween said elements, and an electrical connection between a point ofelectrical symmetry in said input circuit and a point of electricalsymmetry in said duplex electron multiplier for maintaining saidapparatus in electrical symmetry.

vALBERT M. SKELLETT.

